Leading-edge entry/exit management systems make accurate identification possible by utilizing biometric information, but there exists a simple method that slips through even security based on such high-tech. That is, when an individual (e.g., an employee, a resident or the like) authorized by authentication entries through unlocked door, intrusion is allowed by what is called “tailgate” while the door is opened.
A prior art system described in Japanese Patent Publication No. 2004-124497 detects tailgate by calculating the number of persons' three-dimensional silhouettes. The silhouettes are virtually embodied on a computer by the volume intersection method based on the theory that a physical object exists inside a common region (a visual hull) of volume corresponding to two or more viewpoints. That is, the method uses two or more cameras, and virtually projects a two-dimensional silhouette obtained from output of each camera on actual space and then forms a three-dimensional silhouette corresponding to a shape around the whole physical object.
However, in the above system, there is a need to use two or more cameras due to the volume intersection method. The system also captures the face of a person with one of the two cameras, and since the volume intersection method requires putting the detection area (one or more physical objects) in viewrange of each camera, the system cannot form the three-dimensional silhouette while the face or the front is within the viewrange. On account of this, it becomes difficult to follow moving tracks of one or more physical objects in the detection area. Though this issue can be solved by further adding a camera, it results in increase of cost and installation area of the system. In particular, the number of cameras is mightily increased as the number of doors is increased.
Further, the volume intersection method has another issue when a three-dimensional silhouette is formed from overlapping physical objects because it is not technology for separating the overlapping physical objects. By using reference size corresponding to one physical object, the prior art system can detect a state that two or more physical objects are overlapping, but the system cannot distinguish a state that a person and a baggage are overlapping from a state that two or more persons are overlapping. The former does not need to give the alarm, whereas the latter needs to give the alarm. In addition, the prior art system removes noise by calculating differentials between a previously recorded background image and a present image, but even though it is possible to remove a static physical object(s) (hereinafter referred to as “static noise”) such as a wall, a plant, etc, the system cannot remove a dynamic physical object(s) (hereinafter referred to as “dynamic noise”) such as a baggage, a cart, etc.